If you are going to run a fuel line on the outside of the frame rail just make sure it is secure enough with clips and clamps. Most all my Pontiac's have the fuel line run on the outside of the frame and I have yet to hear of any issues ever arising from this type of setup!

It started life inside the frame,and for the most part that is where I run all of mine on coversions,less chance on it getting damaged, If you are running a 400, that is going to require more fuel .I would suggest getting a 3/8 " sending unit assembly for the gas tank & run 3/8 " line all the way to the carb. 5/16 line will cause a fuel starvation issue under hard acceleration.